polymorphic-associations多态关联实例 ruby on rails

这次做新项目的时候，把图片都放在了一个表里，其他表中不再存图片信息，通过多态关联建立表之间的关系。

(1)新建picture表，component表不需要处理

class CreatePictures < ActiveRecord::Migration[5.0]
  def change
    create_table :pictures do |t| 
      t.integer :imageable_id
      t.string :imageable_type
      t.string :name
      t.string :md5
      t.string :url
      t.timestamps
    end 

    add_index :pictures, [:imageable_type, :imageable_id]
  end 
end

（2）关于model的关联

多态关联的文档 http://guides.rubyonrails.org/association_basics.html#polymorphic-associations

A slightly more advanced twist on associations is the polymorphic association.

With polymorphic associations, a model can belong to more than one other model, on a single association.

For example, you might have a picture model that belongs to either an employee model or a product model.

Here's how this could be declared:

class Component < ApplicationRecord
  serialize :hints, Hash

  has_many :pictures, as: :imageable
  accepts_nested_attributes_for :pictures, reject_if: proc { |attributes| attributes['md5'].blank? || attributes['url'].blank? }
end

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true

end

对下拉框的处理

  PICTURE_NAME = {
    '海报图' => 'poster',
    '背景' => 'background',
    '左下角角标' => 'left_bottom_corner',
    '右下角角标' => 'right_bottom_corner',
    '左上角角标' => 'left_top_corner',
    '右上角角标' => 'right_top_corner'
  }

  def picture_name picture_name
    PICTURE_NAME.invert[picture_name]
  end

关于多态关联的Instance Public methods http://api.rubyonrails.org/

accepts_nested_attributes_for(*attr_names) Link

Defines an attributes writer for the specified association(s).

Supported options:

:allow_destroy

If true, destroys any members from the attributes hash with a _destroy key and a value that evaluates to true(eg. 1, '1', true, or 'true'). This option is off by default.

:reject_if

Allows you to specify a Proc or a Symbol pointing to a method that checks whether a record should be built for a certain attribute hash. The hash is passed to the supplied Proc or the method and it should return either true orfalse. 

When no :reject_if is specified, a record will be built for all attribute hashes that do not have a _destroyvalue that evaluates to true. Passing :all_blank instead of a Proc will create a proc that will reject a record where all the attributes are blank excluding any value for _destroy.

:limit

Allows you to specify the maximum number of associated records that can be processed with the nested attributes. 

Limit also can be specified as a Proc or a Symbol pointing to a method that should return a number. 

If the size of the nested attributes array exceeds the specified limit, NestedAttributes::TooManyRecordsexception is raised.

 If omitted, any number of associations can be processed. Note that the :limit option is only applicable to one-to-many associations.

:update_only

For a one-to-one association, this option allows you to specify how nested attributes are going to be used when an associated record already exists. 

In general, an existing record may either be updated with the new set of attribute values or be replaced by a wholly new record containing those values. By default the :update_onlyoption is false and the nested attributes are used to update the existing record only if they include the record's:id value. Otherwise a new record will be instantiated and used to replace the existing one. 

However if the:update_only option is true, the nested attributes are used to update the record's attributes always, regardless of whether the :id is present. The option is ignored for collection associations.

Examples:

# creates avatar_attributes=
accepts_nested_attributes_for :avatar, reject_if: proc { |attributes| attributes['name'].blank? }
# creates avatar_attributes=
accepts_nested_attributes_for :avatar, reject_if: :all_blank
# creates avatar_attributes= and posts_attributes=
accepts_nested_attributes_for :avatar, :posts, allow_destroy: true

http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html

If the hash contains an id key that matches an already associated record, the matching record will be modified:

如果在编辑或者新建component的时候，传的picture相关的参数里带有id，那么这条图片记录会被修改，不会被新建。

member.attributes = {
  name: 'Joe',
  posts_attributes: [
    { id: 1, title: '[UPDATED] An, as of yet, undisclosed awesome Ruby documentation browser!' },
    { id: 2, title: '[UPDATED] other post' }
  ]
}

member.posts.first.title # => '[UPDATED] An, as of yet, undisclosed awesome Ruby documentation browser!'
member.posts.second.title # => '[UPDATED] other post'

（3）controller里的处理

 Nested Parameters

http://guides.rubyonrails.org/action_controller_overview.html#strong-parameters

  private
    def set_component
      @component = Component.find(params[:id])
    end 

    def component_params
      params.require(:component).permit(:release_id, :component_type_id, :name,
                                        :unitary, {pictures_attributes: [:id, :imageable_type, :imageable_id, :name, :url, :md5]},
                                        :remark, component_ids: [], hints: [:left, :mid, :right]
                                        )
    end 

（4）_form.html.erb里的处理

  <div class="image-group">
    <% @component.pictures.each do |p| %>
      <%= hidden_field_tag  'component[pictures_attributes][][id]', p.id %>
      <div class="form-group">
        <%= f.label 'name', class: 'col-sm-2 control-label' %>
        <div class="col-sm-4">
          <%= text_field_tag 'component[pictures_attributes][][name]', @component.picture_name(p.name), class: 'form-control' %>
        </div>
      </div>

      <div class="form-group">
        <%= f.label 'md5', class: 'col-sm-2 control-label' %>
        <div class="col-sm-4">
          <%= text_field_tag 'component[pictures_attributes][][md5]', p.md5, class: 'form-control' %>
        </div>
      </div>

      <div class="form-group">
        <%= f.label 'url', class: 'col-sm-2 control-label' %>
        <div class="col-sm-4">
            <%= image_tag p.url, :width => 300 %><br/>
        </div>
        <div class="col-sm-4">
          <%= text_field_tag 'component[pictures_attributes][][url]', p.url, class: 'form-control' %>
        </div>
      </div>
    <% end %>
  </div>

  <div class="image-group image-create-group">
    <div class="form-group">
      <%= f.label 'name', class: 'col-sm-2 control-label' %>
      <div class="col-sm-4">
        <%= select_tag 'component[pictures_attributes][][name]', options_for_select(Component::PICTURE_NAME,""),
          class: "form-control" %>
      </div>
    </div>

    <div class="form-group">
      <%= f.label 'md5', class: 'col-sm-2 control-label' %>
      <div class="col-sm-4">
        <%= text_field_tag 'component[pictures_attributes][][md5]', nil, class: 'form-control' %>
      </div>
    </div>

    <div class="form-group">
      <%= f.label 'url', class: 'col-sm-2 control-label' %>
      <div class="col-sm-4">
        <%= text_field_tag 'component[pictures_attributes][][url]', nil, class: 'form-control' %>
      </div>
    </div>
  </div>